A Numerical Comparison of Spray Combustion between Raw and Water-in-Oil Emulsified Fuel

Abstract

Heavy fuel-oils, used engine oils and animal fat can be used as dense, viscous combustibles within industrial boilers. Burning these combustibles in the form of an emulsion with water enables to decrease the flame length and the formation of carbonaceous residue, in comparison with raw combustibles. These effects are due to the secondary atomization among the spray, which is a consequence of the micro-explosion phenomenon. This phenomenon acts in a single emulsion droplet by the fast (< 0.1 ms) vaporization of the inside water droplets, leading to complete disintegration of the whole emulsion droplet. First, the present work demonstrates a model of spray combustion of raw fuel. Secondly, the spray combustion of water-in-oil emulsified fuel is exposed to the same burning conditions, taking into account the micro-explosion phenomenon. Finally, the comparison between the results with and without second atomization shows some similar qualitative tendencies with experimental measurements from the literature.drag factor for spherical shape INTRODUCTIONMany cheap, available combustibles like heavy fuel-oil, used engine oil or animal fat can be used as combustibles, in spite of their higher density and viscosity. To burn these combustibles as water-in-oil emulsions has been known to shorten the flame length and to decrease the formation of carbonaceous residues and NOx, in comparison with the raw combustibles [1,2]. These changes are caused by the second atomization of the emulsified combustibles. Indeed, second atomization is the consequence of the phenomenon of micro-explosion upon individual droplets. Micro-explosion means the fast (less than 0.1 ms. [3,4]) vaporization of inside water droplets, enabling to break the initial emulsion droplet up into a group of smaller droplets. Nevertheless, the microexplosion delay, the length and the temperature of the flames of emulsified fuel are required in order to design emulsion-burning boilers and heating devices. Law [5] defines theoretically and physically the limiting temperatures for the onset of micro-explosion in an individual droplet. Nazha et al.[6] make use of a specific droplet model and a probability function to determine to what extent the micro-explosion predicted by the droplet analysis affects the spray development. This work demonstrates a model of spray combustion in order to obtain the fields of local combustion rate and temperature. A steady, 2D numerical simulation of the combustion chamber has been performed. The turbulent flow of gases (standard k-ε model [7]), the combustion within gaseous phase ("eddy-dissipation" model [8]), the transport equations for the 5 different gaseous species (C 19 H 30 , O 2 , CO 2 , H 2 O, N 2 ), and the boundary conditions are defined independently from the raw or emulsified nature of the combustible. In order to model liquid fuel combustion, a Lagrangian model of velocity, heat and mass transfer has been applied to liquid droplets. To model the injection of a spray, droplets are inserted in the computational dom...